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August 2003 Newsletter
The Nuts & Bolts of It
Can’t say the writer is feeling too confident about the Rugby
world Cup. Although the Springboks have got their teeth into the task, we should
see them off. It does however look like the All Blacks have left the sheep to
concentrate on the Rugby.
Corrosion
This month we will continue our discussion of corrosion.
Corrosion of fasteners
varies greatly. There may be a fairly uniform coating of red rust (on either
steel or stainless steel) over the exposed surface, or only a particular area
may be damaged. The formation of deep pits is an example of localised corrosion.
Before specifying preventative measures it is pertinent to identify the type of
corrosion. We covered galvanic corrosion last month, so now we will move on.
Uniform Corrosion
The most "desirable" form of attack, where the damage is uniform
across the entire surface. The rate of attack may be fast, or slow, and the rate
may vary with time.
You may have seen the Botany Town Centre signs, which are plain
unprotected steel. These have an intended rich red patina of rust. Further
rusting will be slow, although the initial rust appeared almost overnight.
Stress Corrosion Cracking
This little understood phenomenon occurs when highly tensile stressed
fasteners are exposed to a corrosion inducing atmosphere. In simple terms
microscopic cracks develop in areas of high stress concentration, and high
surface stresses appear to attract and localize the corrosion process.
The mutually amplifying effects of stress and corrosion cause
inward growth, which progressively reduces the effective cross section of the
fastener until it fails.
Similarly Corrosion Fatigue may result when a dynamically loaded
fastener is simultaneously exposed to corrosive attack Again the combination is
very much more damaging than either acting alone.
Fretting Corrosion
As the name implies, Fretting corrosion is caused
when loaded contact surfaces rub against each other. The movement, usually
vibration, abrades the protective oxide films and prevents their regeneration.
This is particularly true of Stainless Steels.
Pitting
Pitting is caused by rapid attack at small points
on the surface, while the rest of the surface is either unaffected or uniformly
attacked at a relatively slow rate. This type of non-uniform attack is usually
seen on bolt heads, or nuts.
The use of Graphite greases on Stainless
steel is often the cause of pitting, as the carbon removes the protective oxide
coating.
Galvanising is the best preventative for
Pitting.
Crevice Corrosion
Crevice Corrosion, sometimes called Concentration
Cell Corrosion, is an electrochemical type of corrosion. It is initiated when
different areas are exposed to different concentrations of the same electrolyte.
Crevice corrosion is localized in its attack, occurring in crevices, internal
corners, low points, and openings where moisture, dirt and other foreign
materials can accumulate and create a stagnant pocket of concentrated
electrolyte. This is often seen where fully threaded rod is used in place of a
bolt.
 Erosion Corrosion
Metals which depend on a self generated protective
film for their corrosion protection may suffer Erosion corrosion when exposed
high velocity fluid flow. If the flow is turbulent, or if the fluid carries
solids in suspension, the severity of the corrosion is significantly increased.
If ignorance is bliss, how come there aren't more happy people? - Toilet Wall.
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